1. Field of the Invention
The present invention relates to magnetic bearings and, more particularly, to a passive magnetic bearing used to support and stabilize the rotor of a motor-generator.
2. Description of Related Art
An armature and a field winding comprise the primary elements of motors, generators and alternators. In low power applications, the armature rotates through the magnetic lines of induction provided by the stationary field winding. In this configuration, the armature composes the rotor part of the assembly, while the field winding is the stator. This is the design used in automobile alternators because the stator is on the outside of the rotor, and thus can be incorporated into a protective casing to facilitate replacement of the assembly. In high-power industrial applications, the field winding usually rotates while the armature composes the stator.
Regardless of the configuration, the rotor requires a degree of freedom to rotate about is longitudinal axis. Mechanical bearings, such as journal bearings, ball bearings, and roller bearings are commonly used for this purpose. Such bearings necessarily involve friction between the rotating element and the bearing components. This reduces the efficiency of the unit, and the designer must also contend with the attendant problems of heat and wear.
Even non-contact bearings, such as air bearings, involve frictional losses that can be appreciable and, in addition, are sensitive to dust particles. Furthermore, mechanical bearings, and especially air bearings, are poorly adapted for use in a vacuum.
The use of magnetic forces to provides an attractive alternative because, as it provides for rotation without contact, it avoids the aforementioned drawbacks. One such approach uses position sensors to detect incipient unstable motion of the rotating element and then uses magnetic coils in conjunction with electronic servo amplifiers to provide stabilizing forces to restore it to its (otherwise unstable) position of force equilibrium. The foregoing is usually designated as an “active” magnetic bearing, in reference to the active involvement of electronic feedback circuitry in maintaining stability.
Less common than the servo-controlled magnetic bearings just described are magnetic bearings that use superconductors to provide a repelling force acting against a permanent magnet element in such a way as to levitate that magnet. These bearing types utilize the flux-excluding property of superconductors to maintain a stable state by appropriately shaping the superconductor and the magnet to provide restoring forces for displacements in any direction from the position of force equilibrium. Obviously, magnetic bearings that employ superconductors must keep the superconductor at cryogenic temperatures, and this comprises a significant consideration for any design incorporating this type of bearing.
As may be seen from the foregoing, there presently exists a need in the art for a bearing that avoids the shortcomings and problems attendant to using mechanical bearings, but does so without the drawbacks and design limitations associated with active or superconducting magnetic bearings. The present invention fulfills this need in the art.